VI 



.-1 XTENXA TA THE RESPIRA TOR Y ORGA NB 



481 



2. In many Insects with incomplete metamorphosis the holopnenstic condition 

 of the tracheal system is much altered in the larvre by the adaptation to aquatic 

 life. In the aquatic larvae of the Ephcmeridce, Odonata, I'ln;,/,!, rn, for instance, 

 there are no open stigmata, the tracheal system is completely closed, i.e. it is 

 apneustic. The rudiments of the tracheal branches running from the stigmata to 

 the longitudinal trunks are, however, present, but they are empty of air, and appear 



FIG. 330. Machilis maritima, 

 representing the traclieal system of 

 the right side (after Oudemans). 7r, 

 Head ; /, II, III, thoracic segments ; 

 1-10, abdominal segments ; s, stig- 

 mata. 



FIG. 340. Half -developed larva of an 

 agamous unwinged female of Aphis Pelar- 

 gonii. The tracheal system seen from above 

 (after Witlaczil). 7;, Head ; a, antenna ; /, II, 

 III, segments of the thorax ; 1V-XII, segments 

 of the abdomen ; 6j, bo, b 3 , the 3' pairs of legs ; 

 s, the stigmata of the tracheal system. 



in the form of strands (Fig. 343, vf) ; these play an important part in the shedding 

 of the larval tracheae, and then for a time open outwardly. When the last larval 

 stage passes into the imaginal stage these strands become hollow, and the traclieal 

 system becomes holopneustic. All such larvse breathe by means of tracheal gills. 



3. In the majority of the larvte of insects which undergo complete metamorphosis, 

 the stigmata of those segments which in the imago carry wings (meso- and meta- 

 thorax) are closed, but on the other hand one pair of prothoracic stigmata wanting 

 in the imago is usually found. We evidently have here a delayed differentiation of 

 the tracheae which supply the wings and their musculature, referable to the absence 

 VOL. I 2 I 



